The present invention generally relates to slurries of titanium dioxide. More specifically, the present invention relates to concentrated slurries of titanium dioxide and methods for producing concentrated slurries of titanium dioxide.
Titanium dioxide (xe2x80x9cTiO2xe2x80x9d) has many known uses in industry. For example, TiO2 is used as a pigment for many paints. It is also used in other applications such as paper making, plastics applications and coating compositions.
Methods for producing TiO2 are also known in the art. TiO2 is generally produced by hydrolyzing an aqueous solution of titanium sulfate and calcining the hydrolyzate at about 750-1000xc2x0 C., or oxidizing titanium tetrachloride (xe2x80x9cTiCl4xe2x80x9d) at elevated temperatures followed by cooling at temperatures below about 600xc2x0 C. The TiO2 thus prepared is dry or wet ground and then dry milled. One method of dry milling is accomplished by utilizing a micronizer (also sometimes referred to as a fluid energy mill). Utilizing a micronizer typically involves introducing dry TiO2 pigment and a fluid, typically steam, into the micronizer. The output of the micronizer typically contains from about 20 percent of TiO2 to about 50 percent of TiO2, with substantially all of the remaining output composition comprising steam.
In most pigment manufacturing operations the TiO2 in the output from the micronizer is separated from the steam, cooled and packaged as a dry particulate. This finished TiO2 pigment can then later be formed into an aqueous slurry for the production of paints, coatings, and the like. In one process for preparing such aqueous slurries of TiO2, the finished TiO2 pigment is added to a water-dispersant package in a batch mixer having a high shear agitator. The slurry is then stirred until the degree of dispersion required is achieved in the slurry. Typically the dried TiO2 pigment is shipped to a customer and the slurry preparation is performed by the customer. However, it is becoming more and more common for the producer of the TiO2 pigment to produce and sell the pigment in a slurry form, and higher solids (i.e., more concentrated) slurries containing from 60 to 80 percent solids and higher, more particularly comprising at least about 60 weight percent of titanium dioxide, preferably at least about 65 and more preferably at least about 76 weight percent of titanium dioxide, are in increasing favor.
To separate the TiO2 in the micronizer output from the steam in the micronizer output, the output of the micronizer is typically introduced into a cyclonic gas-solids separator (or cyclone), wherein solid TiO2 product falls to the bottom of the cyclone and is removed in the underflow while the steam typically exits from the top in the overflow. The solid TiO2 product is then sent to a silo for packaging and shipping or to be used to produce a slurry as described above. The steam that exits from the cyclone is sometimes referred to as micronizer tailings. These micronizer tailings can be processed to produce TiO2 slurries using known methods such as the one taught in U.S. Pat. No. 4,427,451 issued to Baloga.
The present invention relates in a first aspect to an improved method of producing titanium dioxide slurries. According to the present invention in this first aspect, the output from a micronizer (that is, substantially steam and TiO2) is passed through a condenser to condense the steam and form an aqueous slurry of titanium dioxide, which can in most cases then be further concentrated as desired or which can be sold or used without further concentration, instead of passing the micronizer output to apparatus for separating the TiO2 from the steam, as is currently practiced in the art.
The resultant slurry from the condenser (hereinafter also referred to as the xe2x80x9ccondensate slurryxe2x80x9d) is in a further refinement preferably sent to an agitated storage tank together with dispersant and anti-settling compounds to aid in preventing settling of the titanium dioxide solids out of the slurry, with the amounts of dispersant and anti-settling compounds added being based on the weight of titanium dioxide fed to the micronizer. To help facilitate condensation of the steam/pigment mixture sent to the condenser, a portion of the condensed slurry is in yet a further refinement pumped from the slurry storage tank to a heat exchanger. The output from the heat exchanger is then fed to the condenser to facilitate the condensing of the steam in the condenser. A secondary benefit of this re-circulation of condensate is to keep the condenser from fouling due to deposits of titanium dioxide on the internal plates or tubes of the condenser and thereby lose cooling efficiency. The temperature and volume (flow) of cooled condensate to the condenser are set to match the amount of steam and its temperature so as to ensure that substantially all of the steam and contained pigment are collected from the condenser.
To start the operation without a supply of condensate, sufficient deionized water is fed to the condenser until the level of condensate in the storage tank is adequate to allow for the re-circulation loop described above to be put into operation. At that point the use of deionized water is discontinued. While this results in some initial dilution of the condensate, the amount added is small and would not be expected to significantly impact the overall operation.
The present invention relates in a second aspect to a process of making a concentrated titanium dioxide slurry containing at least about 60, preferably at least about 65 and more preferably at least about 76 weight percent of titanium dioxide in an aqueous medium, wherein a condensate slurry prepared as described above is further concentrated as to the titanium dioxide component of the condensate slurry. Various apparatus and methods are known which can be used for accomplishing the further concentration of the condensate slurry, for example, by membrane filtration as shown by U.S. Pat. Nos. 4,952,317 and 5,014,564 to Culkin and by U.S. Pat. No. 5,622,628 to Trendell et al. The present invention relates in its final aspect to the concentrated slurries produced by such methods.